Electron discharge devices

ABSTRACT

An electron discharge device includes a cathode for producing electrons, a target for collecting said electrons and an electron focusing system for focusing the electrons from said cathode into a beam directed to said target. The focusing system includes a member having an aperture for the passage of the beam and a charge retaining surface surrounding said aperture on which in operation of the tube unfocused electrons from said cathode impinge, said surface having such a coefficient of secondaryelectron emission for the energy of the cathode electrons under the rated operated conditions of the device that unfocused electrons tend to cause said surface to charge to a potential which will focus said electrons to pass through said aperture.

United States Patent Phillips et al.

[54] ELECTRON DISCHARGE DEVICES [72] Inventors: Graham Phillips, London; Dudley Perring, Cookham, both of England [73] Assignee: Electric & Musical Industries Limited, Hayes, Middlesex, England [22] Filed: July 17, 1970 [21] App1.No.: 55,879

[56] References Cited UNITED STATES PATENTS 2,416,303 2/1947 Parker ..313/103 X 2,581,408 1/1952 Hamilton ..315/5.12 2,900,559 8/1959 Ruetz ...315/5.l2 2,645,739 7/1953 Fremlin et al. ..31S/5.l1 2,517,726 8/1950 Skellett ..315/5.12 3,449,617 6/1969 Kreuchen et al. ..315/5.34 2,921,215 1/1960 Birdsall ..313/85 1 Aug. 8, 1972 Fremlin et al ..313/68 R Ruetz ..313/68 R FORElGN PATENTS OR APPLICATlONS Great Britain ..315/5.12 Great Britain ..3 l5/5.11

[57] ABSTRACT An electron discharge device includes a cathode for producing electrons, a target for collecting said electrons and an electron focusing system for focusing the electrons from said cathode into a beam directed to said target. The focusing system includes a member having an aperture for the passage of the beam and a charge retaining surface surrounding said aperture on which in operation of the tube unfocused electrons from said cathode impinge, said surface having such a coefficient of secondary-electron emission for the energy of the cathode electrons under the rated operated conditions of the device that unfocused electrons tend to cause said surface to charge to a potential which will focus said electrons to pass through said aperture.

1 1 Claims, 8 Drawing; Figures PATENTEUMIB' 8 I912 SHEET 1 [IF 4 Invaders Graham II/IF} PuJlcy Parring Illorncys ELECTRON DISCHARGE DEVICES This invention relates to electron discharge devices and particularly to electrostatic means for the focusing of the electron beam in such devices.

With high power devices operating with high voltages, such as amplifier klystrons, problems of insulation arise when electrostatic focusing is employed for maintaining the electron beam against space charge forces. The insulation problem is encountered, for example, in the case of the lead to a focusing member. This lead may pass from the exterior through a metal part of the envelope maintained at the same potential as the cavity walls, and the lead therefore needs to be insulated therefrom. Also, it is necessary where the lead passes through the envelope to provide a vacuum seal.

It is an object of the invention to provide an improved electron discharge device having electrostatic focusing of the electron beam.

According to the invention there is provided an electron discharge device including a cathode for producing electrons, a target for collecting said electrons and an electron focusing system forfocusing the electrons from said cathode into a beam directed to said target, said focusing system including a member having an aperture for the passage of the beam and a charge retaining surface surrounding said aperture, said member being positioned to intercept unfocused electrons of said beam whereby said charge retaining surface acquires a charge which co-operates in the focusing of said beam.

In order that the present invention may be clearly understood and readily carried into effect, it will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 illustrates diagrammatically a longitudinal sectional view of an amplifier klystron utilizing the invention,

FIG. 2 is a view on the line IIII of FIG. 1 and illustrates a mounting arrangement for a focusing member,

FIG. 3 illustrates diagrammatically a longitudinal sectional view of a travelling wave tube,

FIG. 4 is a plan view of a focusing member shown in FIG. 3,

FIG. 5 is a plan view of another embodiment of focusing member,

FIG. 6 is a sectional view on the line VI-VI of FIG.

FIG. 7 is a longitudinal sectional view of a further embodiment of a focusing member, and

FIG. 8 is a longitudinal sectional view of yet a further embodiment of a focusing member suitable for use in carrying out the invention.

Referring to FIG. 1, the klystron which is illustrated comprises an electron gun 1 including a thermionic cathode, four resonant cavities 2, 3, 4 and 5 and a collector electrode 6, arranged in that order along the axis of the klystron. Each of the cavities is formed by two transverse copper walls and by part of the copper envelope of the klystron, the transverse walls being denoted by the references 7 and 8 in the case of each cavity and the copper envelope of the klystron being denoted by the reference 9. The walls 7 and 8 are formed with drift tubes 10 and 11 in known manner,

2 2 is the input cavity and high frequency signals can be fed to this cavity by way of a coupling loop 13. The cavity 5, on the other hand is the output cavity and is coupled to an output waveguide 14 through a dielectric window 15.

To enable the electrons from the gun I to be focused so as to form a concentrated axial beam which passes through the cavities 2, 3, 4 and 5 and which is finally collected by the collector target electrode 6, electrodes 16, 17 and 18 are provided between each pair of cavities. Each transverse wall 7 and 8 and its related drift tube 10 is at the same potential as the envelope 9 to which is it connected. The members 16, 17 and 18 are at potentials different from that of the transverse walls 7 and 8 and form therewith a focusing system comprising a series of electrostatic lenses. Each of the members 16, 17 and 18, hereinafter referred to as focusing members, is mounted by means of a ceramic insulator 19 to insulate the focusing members from the envelope, the leakage path resistance being of the order of 200 megohms or greater. The insulator illustrated in this example fon'ns the subject of our U.S. Pat. No. Each focusing member and its mounting insulator is of the same construction and the following description of the electrode 17 and its respective insulator is therefore applicable to all such focusing members.

As can be seen from FIGS. 1 and 2, the member 17 is planar having a central aperture for passage of the electron beam. Lips 21 and 22 are provided on both edges of the member 17 at four angularly spaced positions and cooperate with four tongues 23 on the insulator 19 to locate and mount the member as described in the aforesaid specification.

It will be appreciated that although in the example described above, the transverse walls 7 and 8 are used as electrodes which co-operate with the focusing members 16, 17 and 18 to provide electrostatic lenses, separate electrodes may be utilized to co-operate with the or each focusing member.

Each focusing member is made of, or is coated with, a material which exhibits secondary electron emission characteristics. If the focusing members are to be used at cathode potential, they may be made of copper and have a coating of a substance whose secondary emission coefficient is below unity for the energy of the impinging electrons, for example soot tantalum carbide or titanium dioxide. If the focusing members are to be used at a potential which is positive with respect to cathode potential they may be made of, or coated with, a substance having a secondary emission co-efficient which has a portion greater than unity, for example copper, in which case the operating conditions of the tube may be chosen such that the member stabilizes at the second cross-over potential. In the cases where the secondary emissive material comprises electrically insulating material, an electrically conductive underlayer is provided to prevent an uneven distribution of potential over the surface.

Referring now to FIG. 3, there is represented a travelling wave device having an electrically conducting envelope 30. Three cavities 31, 32 and 33 are coupled together by windows or apertures 34 and 35 in the transverse walls of the cavities. Thecavity 31 is the input cavity and cavity 33 is the'output cavity (the input and output means being omitted for simplicity).

The cavities 31, 32 and 33 are coupled respectively by apertures 36, 38 and 39 to an electron beam passing axially down the tube from a cathode 40 to a collector 41. The cathode 40 is connected to the negative side of a power supply by a lead passing through an insulating bushing 42 while the collector 41 may be connected to the envelope 30 which is earthed and connected to the positive side of the power supply. Spaced along the path of the electron beam are focusing cavities 43 and 44 containing focusing members 45 and 46 respectively. The Focusing members may be secured to the focusing cavities by brazing for example. In the travelling wave tube shown, focusing electrostatic lenses are formed between the edges of the focusing cavities and the focusing members when they become charged due to secondary emission. In this case, the edges of the focusing cavities act as apertured electrodes, but it will be appreciated that separate apertured electrodes could be provided to co-operate with the focusing members to provide electrostatic focusing lenses.

FIGS. 4 to 8 show various constructions for the focusing members which may be employed in a travelling wave tube. In FIG. 4 which shows the focusing member 45 of FIG. 3, the electrode is in the form of an annular disc of insulating material. The crosshatched portion adjacent the central aperture indicates the secondary electron emissive area. In order to increase the length of the leakage path from the conducting wall of the focusing cavity to the secondary emissive portion of the electrode, the electrode may take the form shown in FIGS. and 6 in which an annular disc 47 of insulating material is provided on each major surface with a plurality of concentric grooves 48, 49 and 50. Alternatively, a single groove in the form of a spiral may be formed on each surface of the disc. The cross-hatched portion shown in FIG. 5 again indicates the secondary electron emissive area adjacent the aperture in the disc. The focusing member shown in FIG. 7 takes the form of cylinder 51 of insulating material having on the inside thereof an annulus 52. The portion of the annulus 52 adjacent the aperture is secondary electron emissive in a similar manner to the disc 45 of FIG. 4. The inner surface of cylinder 51 is provided with a plurality of grooves 53 to 56 on one side of the annulus 52 and a plurality of grooves 57 to 60 on the other side of the annulus 52 in order to increase the length of the leakage path from the secondary emissive area of annulus 52. The focusing member shown in FIG. 8 comprises a cylinder 61 of insulating material having an inner annulus 62 and an outer annulus 63. On one side of annulus 62 there is provided in the inner surface of cylinder 61 a plurality of grooves 64 to 67 and on the other side of annulus 62 there is provided a plurality of groves 68 to 71. The outer surface of cylinder 61 is similarly provided with a plurality of grooves 72 to 79. The portion of annulus 62 adjacent the aperture is secondary electron emissive in a similar manner to the disc 45 of FIG. 4. The separate grooves in the focusing members shown in FIGS. 7 and 8 may be replaced by helical grooves.

In the focusing members of FIGS. 4 to 8, the secondary electron emissive areas have an electrically conductive under-layer in order to prevent an uneven distribution of potential thereon.

When the discharge device is being warmed up prior to operation, electrons from the beam impinge upon each focusing member which thus becomes charged due to secondary emission and because it is insulated electrically it is able to maintain this charge all the while the device is operating. The charges build up until equilibrium conditions are attained. The potentials on each focusing member will dependupon the surface material thereof. If it is a material whose secondary emission coefficient is below unity for the energy of the impinging electrons then the electrodes will stabilize at cathode potential. If it is a material whose secondary emission co-efiicient has a portion greater than unity it will stabilize at the second cross-over potential if the electron beam velocity is sufiicient. Second cross-over potential may be, for example, 8 KV, so that if the cathode potential is 12 KV then the focusing electrodes stabilize at 4 KV. The other parameters of the discharge device are so chosen that the potentials attained by the focusing electrodes will focus the beam which will then pass through the central apertures in the focusing members.

In practice, each focusing member is maintained at its operating potential by stray electrons impinging upon the secondary emissive surface of the focusing member, thus maintaining the focusing of the beam.

It will be seen that the invention provides some important advantages. Firstly, because no electrical connections are required to the focusing members, electron discharge devices according to the invention are simpler and cheaper to manufacture. Secondly, in operation, because the focusing members are not connected to the H.T. supply, the capacity of the focusing members as seen by the H.T. supply is zero. This reduces power losses and is also especially advantageous when employing cathode modulation.

What we claim is:

1. An electron discharge device including a cathode for producing electrons, a target for collecting said electrons and an electron focusing system for focusing said electrons from said cathode into a beam directed to said target, said focusing system including a member having an aperture for the passage of the focused beam and a charge retaining surface surrounding said aperture, an electrode adjacent to said member said member being positioned to intercept unfocused electrons of said beam whereby said charge retaining surface by virtue of secondary emission to the adjacent electrode acquires a charge relative to the adjacent electrode which co-operates in the focusing of said beam.

2. A device according to claim 1 in which said charge retaining surface comprises a layer of secondary electron ernissive material coated on said member.

3. A device according to claim 1 in which said focusing system includes an apertured electrode having an external connection for maintaining said electrode at an applied potential.

4. A device according to claim 1 in which said apertured member comprises insulating material.

5. A device according to claim 2 in which said layer is conductive and is further coated with secondaryelectron emissive material.

6. A device according to claim 1 in which said member is a conductive member coated with secondary-electron emissive material, and electrically insulated from other electrodes of the device.

7. A device according to claim 6 in which said apertured member has no external lead connected to it.

8. A device according to claim 2 in which the co-efficient of secondary-electron emission of said surface under the rated operated conditions is less than unity.

9. A device according to claim 2 wherein said secondary electron emissive material is soot, tantalum carbide or titanium dioxide.

10. A device according to claim 1 wherein said surface of said apertured member is made of a material whose co-efficient of secondary-electron emission is greater than unity under the rated operated conditions.

11. An electron discharge device having a cathode for producing electrons, a target for collecting said electrons, first and second resonant cavities including drift tubes positioned between said cathode and said target, focusing means between said cavities for focusing said electrons from said cathode into a beam directed to said target, said focusing means comprising a member having an aperture for the passage of the focused beam and a charge retaining surface surrounding said aperture, said member being positioned to intercept unfocused electrons of said beam whereby said charge retaining surface acquires a charge which cooperates with at least one of said drift tubes to produce an electron lens which focuses said beam. 

1. An electron discharge device including a cathode for producing electrons, a target for collecting said electrons and an electron focusing system for focusing said electrons from said cathode into a beam directed to said target, said focusing system including a member having an aperture for the passage of the focused beam and a charge retaining surface surrounding said aperture, an electrode adjacent to said member said member being positioned to intercept unfocused electrons of said beam whereby said charge retaining surface by virtue of secondary emission to the adjacent electrode acquires a charge relative to the adjacent electrode which co-operates in the focusing of said beam.
 2. A device according to claim 1 in which said charge retaining surface comprises a layer of secondary electron emissive material coated on said member.
 3. A device according to claim 1 in which said focusing system includes an apertured electrode having an external connection for maintaining said electrode at an applied potential.
 4. A device according to claim 1 in which said apertured member comprises insulating material.
 5. A device according to claim 2 in which said layer is conductive and is further coated with secondary-electron emissive material.
 6. A device according to claim 1 in which said member is a conductive member coated with secondary-electron emissive material, and electrically insulated from other electrodes of the device.
 7. A device according to claim 6 in which said apertured member has no external lead connected to it.
 8. A device according to claim 2 in which the co-efficient of secondary-electron emission of said surface under the rated operated conditions is less than unity.
 9. A device according to claim 2 wherein said secondary electron emissive material is soot, tantalum carbide or titanium dioxide.
 10. A device according to claim 1 wherein said surface of said apertured member is made of a material whose co-efficient of secondary-electron emission is greater than unity under the rated operated conditions.
 11. An electron discharge device having a cathode for producing electrons, a target for collecting said electrons, first and second resonant cavities including drift tubes positioned between said cathode and said target, focusing means between said cavities for focusing said electrons from said cathode into a beam directed to said target, said Focusing means comprising a member having an aperture for the passage of the focused beam and a charge retaining surface surrounding said aperture, said member being positioned to intercept unfocused electrons of said beam whereby said charge retaining surface acquires a charge which co-operates with at least one of said drift tubes to produce an electron lens which focuses said beam. 